The Women's Health Initiative (WHI) is divided into a clinical study and an observational study. The clinical study will evaluate the benefits and risk of hormone replacement therapy, dietary modification, and supplementation with calcium/vitamin D (CaD) on the overall health of postmenopausal women. Health will be assessed based on quality of live measurements, cause-specific morbidity and mortality, and total morality. The goals of the observational study are to improve risk prediction of CHD, breast cancer, colorectal cancer, fractures, and total mortality in postmenopausal women, to create a resource of data and biologic samples which can be used to discover new risk factors and/or biomarkers for disease, and to examine the impact of changes in individual characteristics on disease and total mortality.